1. Field of the Invention
The present invention relates to an electrophotosensitive material which is used in image forming apparatuses such as electrostatic copying machine, facsimile, laser beam printer and the like.
2. Description of the Prior Art
In the image forming apparatuses, various organic photosensitive materials having a sensitivity within a wavelength range of a light source used in the image forming apparatuses can be used. Recently, these organic photosensitive materials have widely been used because of easier production than that in the case of a conventional inorganic photosensitive material, various selective materials for photosensitive material, for example, electric charge transferring material, electric charge generating material, binding resin, etc., and high design freedom.
The organic photosensitive material includes, for example, a single-layer type photosensitive material wherein an electric charge transferring material and an electric charge generating material are dispersed in the same photosensitive layer, and a multi-layer photosensitive meterial comprising an electric charge generating layer containing an electric charge generating material and an electron transferring layer containing an electron transferring material, which are mutually laminated.
The single-layer type photosensitive material has attracted special interest recently by the following reasons. That is, the single-layer type photosensitive material can be easily produced because of its simple structure and film defects can be inhibited on formation of layers and, furthermore, optical characteristics can be improved because of less interface between layers. The single-layer type photosensitive material can be used in any of positive and negative charging type apparatuses, but is exclusively used in a positive charging type apparatus in view of characteristics of constituent materials of the photosensitive material.
An image forming apparatus using the single-layer type photosensitive material has the following advantages and a market for the image forming apparatus has been cultivated.
(1) High image quality is attained because turbulance of move of electric charges between the electric charge generating layer and electric charge transferring layer occurs, like a multi-layer type one.
(2) When using in a positive charging type pparatus, ozone which is detrimental to the human body is hardly evolved.
To meet requirements to an image forming apparatus, such as high speed and energy saving, which have recently been increased more and more, the sensitivity of a conventional single-layer photosensitive material has become insufficient at present.
When using a conventional positive charging single-layer type electrophotosensitive material in an image forming apparatus having a high process speed, which is capable of taking 50 or more copies of a A4-size original, a dose of exposure light is small and a time required for a photosensitive material drum to move from the exposure position to the developing position is short, thereby causing a phenomenon that a developing process is attained during potential attenuation due to exposure.
As a result, the following problems occur. That is, an excess dose of light is required to lower a light potential of the photosensitive material and a change in light potential due to an environmental change increases because of the potential attenuation and, furthermore, the degree of fatigue in the photosensitive material due to repeating of exposure and development becomes severe. Consequently, it becomes difficult to obtain a good image.